Bath control

ABSTRACT

Addition of aluminum chloride to the bath of an aluminum chloride cell for electrolysis of aluminum chloride responsive to measurement of the concentration of aluminum chloride in the cell bath. Total effective cell resistance is the preferred indicator of the aluminum chloride concentration in the bath.

United States Patent [1 1 Nov. 12, 1974 Haupin I BATH CONTROL [75] Inventor: Warren E. Haupin, New

2,919.234 12/1959 Slutin .t 204/67 FOREIGN PATENTS OR APPLICATIONS 687,758 2/1953 Great Britain 204/67 272,246 11/1927 Great Britain 204/67 Pittsburgh, Pa. Primary Examiner-lohn H. Mack Flled? p 1972 Assistant Examiner-D. R. Valentine y APPL No; 241,607 Attorney, Agent, or FirmAbram W. Hutcher; John P.

' r Taylor [52] US. Cl. 204/67 51 Int.Cl ..C22d3/l2 ABSTRfACT [58] FieldofSearch 204/67 OfulummumChlondewthe blllholanflluminum chloride cell for electrolysis of aluminum chlo- [56] References Cited ride responsive to measurement of the concentration of aluminum chloride in the cell bath. Total effective UNITED STFATES PATENTS cell resistance is the preferred indicator of the nlumi i: num chloride concentration in the bath. runo e 3,573,179 3/1971 Dirth et a1 204/243 R X 10 Claims, 6 Drawing Figures E k g m an. t Em u m h. u.

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x x 5. 0 X \X c 4. 8 x I Y 4.6- \z 4.4- x

4 0 l l l l l l l l TIME, M/NUTES PATENTLDmv 1 2mm 3.847, 761

voLrs CELL I/OLTZ SHEET 10F 4 WITH 0000 CIRCULATION 3 AND LOW CURRENT DENSITY WITH AVERAGE CONDITIONS WITH. LOW CIRCULATION AND HIGH CURRENT DENSITY BATH IR ans F/LMIR I I l I I l I I I 0 3 4 5 6 7 a 9 l0 mp A/Cl;

PATENIE Luv 1 2mm 3.847.761 SHEEI 0F 4 POWER SOURCE f v /2 I j I /4 FIG. 6.

POWER SOURCE POWER SOURCE J 2 J 28 3: l

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to addition of aluminum chloride to the bath of one or more electrolytic cells for productionof aluminum therefrom by electrolysis of aluminum chloride. More particularly, it relates to making the addition of aluminum chloride to the bath responsive to the concentration of aluminum chloride in the bath, preferably as indicated by total effective cell resistance.

2 Description of the Prior Art Production'of aluminum by electrolysis of aluminum chloride has not yet become commercial. It is likely that the main reason for this is that the Hall process for reduction of alumina has been the most economical method developed to date. One of the problems associated with use of an aluminum chloride process is the difficulty of employing a bath made up of the proper constituents in amounts suitablefor efficient and ecocalculated amount of aluminum chloride to the bath at a regular time interval based on assumed-current efficiency or at such time as he senses in some mysterious manner that the bath requires more aluminum chlorideLAs already indicated, this rather haphazard addition of aluminum chloride to the bath has meant that to date no satisfactory method of obtaining an optimum amount of aluminum by electrolysis of aluminum chloride has been found, at least not'one which has proved commercially efficient.

SUMMARY OF THE INVENTION I have found, after extended investigation, that an aluminum chloride cell operates most efficiently when the amount of aluminum chloride therein is maintained reasonably close to a pre-planned optimum operating level. If substantial variations from the desired optimum operating level are encountered, difficulties of operation occur. For example, as the concentration of the aluminum chloride bath is gradually depleted due to electrolytic conversion of the aluminum chloride to aluminum, the electrical conductivity of the electrolyte increases, thereby causing the voltage to fall. Eventually a point is reached where a concentration polarization at the cathode or cathodes causes the voltage to rise. This is because convection and diffusion at this point are no longer able to bring aluminum ions to the cathode surface at a sufficiently rapid rate. This depletion of aluminum ions at the cathode results in a cathode overvoltage which raises the counterelectromotiveforce (CEMF) until the next cations in the electrochemical series, for example, those of an alkali metal such as sodium, potassium, or lithium, plate out. As a result',-the'cathode, conventionally made of 2 graphite, is attacked by the alkali metal by a process called intercalation, which is detrimental to the cell. Thus, it is desirable to operate at a predesignated optimum level such as mentioned hereinabove to prevent reaching a sufficiently low concentration of aluminum chloride for an attack on the cathode to take place. The desirable optimum operating level for concentration of aluminum chloride in the bath is above a point at which the alkali metal ions deposit and the voltage begins to rise rather suddenly. On the other hand, if the aluminum chloride concentration is greater than the optimum value, current efficiency is lowered. Accordingly, when I find the aluminum chloride concentration in the bath to be below a desired optimum operating level, I add more aluminum chloride to the bath, and, if above the optimum level, I add less or no aluminum chloride.

While the aluminum chloride concentration may be measured by quantitative analysis of a bath sample, this is rather time consuming and the concentration may often become too low, or undesirably below the optimum operating level, before detection. Accordingly, it is desirable to use quicker methods of analyzing for the aluminum chloride concentration of the bath so that a quick response in adding aluminum chloride to the bath can be made to return the concentration to the desired optimum operating level when it departs there-- from to a greater extent than desired.

One method of doing this is to measure the total effective resistance of the cell or total effective cell resistance. This can be done by employing conventional electrical measuring devices to measure voltage and amperage. The total effective resistance (R of the cell is defined by formula R, [E-(NE)]/I, in which Eis the total cell voltage, N the number of electrolytic cell compartments and E the CEMF of each compartment cell and I is the current in amperes. At optimum operating level, E is generally between 1.9 and 2.0 volts. An exact measurement may be made of this fig ure, if desired, for example by current interruption. By current interruption, I mean that the value NE can be read while the current is interrupted momentarily. In bipolar cells, a plurality of compartments are employed, the compartments being formed usually by a plurality of bipolar electrode plates stacked between a terminal anode and a terminal cathode. For a monopolar cell N l.

The total effective cell resistance (R,,,) is made up of at least four factors, viz., (1) resistance of electrical leads and joints thereof, carrying current to the electrodes in the cell, a resistance which is readily measurable, (2) bath resistance, which decreases as concentration of aluminum chloride decreases, (3) resistance of chlorine 'bubbles in the bath and a bubble layer of chlorine which form at the anode and which first decreases as aluminum chloride concentration decreases, probably because the bubbles become larger as the concentration decreases and therefore rise more rapidly and get out of the way more rapidly and then increases as large bubbles start to cling to the anode, blocking current flow, and (4) an apparent resistance resulting from the CEMF rising as cathode overvoltage increases at low aluminum chloride concentrations, whilethe calculation of the apparent cell resistance (R assumes the value of E, the CEMF, to be constant. We have found that a value of 1.95 volts can usually be used for E with good control results. This results in both cell voltage at constant current and apparent cell resistance having the shape shown in FIG. 2, which will be described hereinbelow in connection with the description of the drawing. Thus, the aluminum chloride level may also be measured by cell volts at constant current. However, apparent cell resistance is relatively independent of current so that the normal current fluctuations do not adversely affect measurement of aluminum chloride. If the voltage applied to the cell is held constant, the current flow may be used to indicate the aluminum chloride concentration in a similar manner, but accurate control of the voltage applied to each cell is costly, as is accurate current control.

It is also possible to determine the aluminum chloride concentration by measuring the bath resistivity, for example, by using a dip cell connected to aconductance bridge conventionally used for other purposes, to measure the conductance of the bath, and converting the conductance to the resistivity which is the reciprocal of the conductance of the bath.

Of the foregoing alternatives, 1 have found the preferred method of measuring the concentration of aluminum chloride in the bath to be measuring the total effective resistance of the cell.

One of the surprising things about the accuracy of the measurement of total effective resistance of the cell, is its reliability in measuring aluminum chloride concentration of the bath when so many factors are to be taken into consideration in computing the total effective resistance or total voltage of the cell. This is further surprising because of the fact that the aluminum chloride concentration in the bath generally ranges from only about 2 to percent or so of the total composition of the bath, the remaining amount of the bath being the alkali metal or alkaline earth metal halide salt in which the aluminum chloride is dispersed.

According to my invention, to control aluminum chloride in the bath, the effective bath resistance may be calculated by the aforementioned equation R,,. [E(NE)]/l and compared with a desired optimum operating level or set point of resistance R. This desired optimum operating resistance level or set point R may be established by chemical analysis of bath samples with simultaneous measurement of current and voltage to calculate R over a broad range of operating levels of aluminum chloride concentration. In other words, the total effective cell resistance or total voltage at constant current may be correlated with aluminum chloride concentration in the bath determined by chemical analysis. If a slow drift in R in the above equation at constant percent aluminum chloride occurs, which is possible due to changes in other components of bath composition, temperature or the like, these changes may be easily corrected for, since they are comparatively slow, by changing the resistance set point in response to periodic chemical analysis of the bath to supplement the measurement of the aluminum chloride concentration of the bath by measuring the total effective resistance or total voltage across the cell. As mentioned previously, one advantage of using the total effective cell resistance as a measurement is that the voltage fluctuations caused by current fluctuations do not affect the determination since conversion to the total effective resistance substantially eliminates any fluctuations over the normal current fluctuation range.

According to one embodiment of the invention, which may be called the on-off control method, a feeder may be turned on when the measured total effective resistance (for example, as calculated by the above-given formula R,, [E-(NE)]/l from measurement by a digital voltmeter and ammeter joined to a computer or the like) is less than a predetermined optimum operating level. The feeder rate may be set to supply aluminum chloride to the bath at a rate in excess of that required to maintain the desired optimum concentration of aluminum chloride in the bath, and, when the total effective resistance becomes higher than that indicated for the desired optimum operating level for aluminum chloride concentration, the feeder may once again be turned off until the total effective resistance drops to the desired level.

According to an alternate embodiment of the invention, three feed rates may be used. For example, as a normal or standard feed rate a set amount of aluminum chloride may be employed as determined to be from past experience the most efficient for maintaining optimum concentration of aluminum chloride in the bath. This normal or'standard feed rate in lbs/sec. represented here by F may be determined by the equation F kNlC where k is 1.015 X 10 as calculated from the faraday and weight equivalents of AlCl N equals the number of compartments in the cell, 1 equals the current in amperes, and C equals an assumed current efficiency in percent known from operating experience. The second feed rate in this embodiment is a high feed rate, for example, a rate of 5 to 20 percent in excess of the normal or standard rate, and the third rate is a low rate, specifically, a rate, for example, of 5 to 20 percent below the normal rate. For example, using R as the total effective resistance and calculating from the equation R,,, Rt (KR), where K is a number selected between 0.001 and 0.01, and R is the effective cell resistance at the optimum aluminum chloride concentration, if the calculated total effective resistance falls within this rate, the aforementioned normal rate of feeding is used. If, however, for example, according to this embodiment, R,,, R-(KR), the fast rate is used, and, if R,,, R+(KR) the slow rate is used.

A third embodiment of the invention is a proportional control method in which the feed rate for aluminum chloride is made proportional to the cell current and equal to the cell requirement at an assumed current efficiency C in percent when R R. According to this system the feed rate is increased, or decreased in the case of negative values, proportional to RR,,,. For example, the feed rate F in lbs/sec. may be represented as F 1.015 X 10 NlC,,{l.0 [(RR,,,)K]}, where I is the current in amperes, C, the most recent measured current efficiency in percent, and K a proportional band constant adjusted by experience to give rapid response without over correction.

Thus, it can readily be seen that, by using any of the foregoing systems or embodiments, I am able to maintain a sufficiently constant concentration of aluminum chloride in the cell bath to enable a cell or series or line of cells to operate at a maximum efficiency, thereby avoiding undue variations from a desired predetermined optimum operating level such as might, on the high side, lower current efficiency, or, on the low side,

promote cathode overvoltage or a destructive so-called cathode effect manifest by a sudden rise in voltage dition of aluminum or effective resistance when the concentration of aluminum chloride in the cell bath becomes too low.

It can be seen from the foregoing facts that according to my invention, the regulation of the amount of aluminum chloride added to the cell electrolyte may be adapted as desired to fit the needs of the particular situation encountered. For example, in one embodiment, aluminum chloride may be added at a desired predetermined or pre-calculated optimum or most efficient operating rate until the concentration of aluminum chloride in the cell bath, for example, as determined by a measure of the total effective cell resistance or total cell voltage at constant current, departs a prescribed amount oneither side of a predetermined or preestablished optimum operating level. At this point, if the departure is on the upper side, addition of aluminum chloride may be stopped altogether or the rate of addition decreased. If the departure is on the lower side at this point, the addition of aluminum chloride may be increased, or, if a batch system is used in which a batch of aluminum chloride is added at the start of the opera-.

tion and no more added-until this departure, addition of another batch may be made, or addition of aluminum chloride may then be started at a pre-planned rate and continued at least until the measured total effective cell resistance or voltage drop once again approaches the predetermined or pre-established optimum operating level. p

It is not critical according to the invention how often the total effective resistance, for example, as represented by R is measured, although, the more often this is done, in general, the more efficient the operation, as the less chance there is for the concentration of aluminum chloride in the bath to vary appreciably or substantially from the pre-planned optimum operating level or from a point which tends to give the most efficient operation of the cell and conforms most closely to its needs. For example, readings might be made at anywhere from second to 3 minute intervals.

One of the advantages of my invention is that the control system may be made substantially automatic, for example, by use of a computer tied in with an aluminum chloride feeder. According to such a system, the computer can directly read the total voltage across the cell or calculate, from this reading and a reading of the current at whatever intervals are desired, the total effective resistance, in accordance with the formula given hereinabove. Thus, the computer can relay a signal to the feeder when the concentration of aluminum chloride indicates that more or less aluminum chloride is to be added, or addition of aluminum chloride started or stopped, according to any one of the preceding addition plans, using the aforementioned predetermined optimum operating level of aluminum chloride and optimum operating total effective cell resistance and a designated amount of departure therefrom as the criteria for making adjustments in the rate or amount of adchloride to the cell bath by' the feeder.

The AlCl being fed to the cell may be either solid, liquid, dissolved in bath, or gaseous and may, if desired, be transported either solid or gaseous in dry gas such as Cl CO N etc. AlCl may be added above the bath or under the bath in the cell or in an absorption chamber external to the cell through which bath from the cell circulates.

A computer may also be employed to take care of correcting certain situations encountered during electrolysis of aluminum chloride which are not accurately predictable. For example, the same computer may be employed to monitor the effect of a-change in feed rate on the changes in total effective cell resistance. If the total effective cell resistance changes in the opposite direction from the predicted, that is, if it does not begin to approach the predetermined optimum operating level for concentration of aluminum chloride when more or less aluminum chloride is added in response to its departure from a predesignated efficient or opti .mum operating level, for example, as determined by whether the measured resistance is less than or more than a predetermined optimum value, but continues to depart either further upwardly or downwardly from said optimum level or value, both the computer and an operator know that the total effective resistance has passed the minimum for safe operation of the cell on the standard curve of total effective cell resistance or total voltage across-the cell versus concentration of aluminum chloride in percent, and is operating on the other side or reverse side of the standard operating slope for R or total effective cell resistance or voltage, that is, has reached the point where an increase in total effective cell resistance or voltage drop begins and continues rapidly, as explained hereinabove, this point corresponding to the point where polarization occurs at the cathode and, if allowed to continue, might be destructive to the cathode. When this is sensed by the computer, itrelays a signal to a feeder, which adds aluminum chloride at the maximum possible feed rate, and, if desired, sounds an alarm to which it is operably connected in a known manner so that whatever corrections deemed appropriate may be taken by an operator. For example, cell current may be interrupted momentarily to avoid cathode attack before the aluminum chloride concentration can be replenished to solve the problem. If the total effective cell resistance or voltage continues to rise, the cell current may be interrupted automatically when a predetermined maximum value is reached and remain off until appropriate corrective measures have been taken so that the automatic system may be continued.

On the input side, a computer-cell interface of such a system may consist, for example, of a group of capacitors which transfer voltage and current information through a conventional analogto-digital converter to a computer. On the output side of the computer, the system may consist, for example, of contact closures to control feeders and adjust current. The input capacitors provide isolation between the signal source and the computer and remove signal noise, which is typified by extreme or radical fluctuations in current and/or voltage. By varying a resistance value, a signal maybe averaged over any desired period and at the same time the greatest weight given to the data most recently received and used by the computer. Resistors may also be used to attenuate the signal to a level best suited for the analog-to-digital converter.

In operation such a computer periodically scans current and voltage in sequence to each cell. To do this it sends a signal to an actuating device which causes the capacitor associated with each cell and a current measuring device to be switched'from the current measuring device and cell to the analog-to-digital converter for the period required for a reading, which is generally less than 1/200 second. Signals enter the computer from the converter wherein the above-described calculations are made. The computer then sends an electrical impulse or a series of electrical impulses to a feeder control on each cell requiring correction. These impulses increase or decrease the feed rate as required, for example, by activating a stepping switch.

DESCRIPTION OF THE DRAWINGS For a better understanding of my invention, reference will now be made to the drawings, in which:

FIG. 1 shows a representative breakdown in graphical form of the voltage components on a percompartment basis of an aluminum chloride cell for production of aluminum.

FIG. 2 plots representative curves of cell volts per compartment and effective resistance per square inch of electrode surface per compartment against percent aluminum chloride in a bipolar cell where the current density is maintained at approximately amp/m The curve plotting cell volts versus percent aluminum chloride in FIG. 2 shows an abrupt increase in voltage for average conditions at a concentration of aluminum chloride less than about 2 percent. Such an abrupt increase in cell voltage can generally be avoided by maintaining the aluminum chloride concentration above about 4 percent by weight. With good conditions (i.e., relatively pure aluminum chloride and the bath at appropriate temperature) the abrupt increase in voltage will occur at a lower aluminum chloride concentration, permitting safe operation as low as 1.5 percent .aluminum chloride.

FIG. 3 is a graphical representation showing, for an approximately 45 minute period of operation of an aluminum chloride cell, how, according to the present invention, the concentration of aluminum chloride in a cell bath may be maintained at near an optimum operating level by addition of aluminum chloride when the measured resistance departs more than a desired amount from said optimum .operating level. FIG. 3 includes a graphical plotting of both percent aluminum chloride and resistance versus a short part of the duration of operation of a respective cell in time (minutes). As indicated on the graph, both resistance and percent AlCl were brought back up to an optimum operating level of about 5.3 in this particular instance for the percent AlCl and the resistance (ohms X 10 of 2.3.

FIG. 4 is a schematic representation or diagram showing a computer employed to read the total voltage drop across electrolysis cells or total effective cellv resistance and respond to such reading by sending commands to feeders for cells according to their individual needs when the readings depart beyond the desired or set deviation from that indicating a predesignated optimum operating level for concentration of aluminum chloride in the cell bath.

FIG. 5 is a schematic depiction of a representative batch type feed control relay system employable with the feed control arrangement of FIG. 4 and FIG. 6 depicts a continuous, variable speed feed arrangement for a relay system employable with the feed control arrangement of FIG. 4.

In the drawings, the various components of the schematically represented computer relay system are conventional. What is shown are representative ways in which a computer may be used to read cell current, voltage and resistance according to the aluminum chloride concentration control process of the invention. Readings may be taken at desired intervals, for example, every 5, l0 or 15 seconds, each reading being a eumulative average of the measurements taken over the preceding time interval. In the drawings, each cell may comprise several compartments in a bipolar electrode arrangement. In using a filtered input, one second is used for a representative time constant of the RC circuits shown in FIG. 4, as described in detail hereinafter. This time constant value may be changed, if desired, and is subject to adjustment to prevent any deterioration in control. The scan for each reading of the average voltage drop or average total effective resistance for each cell consumes considerably less time than the RC constant. Thus, for all intents and purposes, the readings of volts and amperes are substantially simultaneous for any cell in a particular potline (potline being the conventional designation for a series of cells).

Referring now to FIG. 4, a digital computer 1 is employed to read the total voltage drop across each of a plurality of electrolysis cells 2 to Zn and to measure the current flowing through each cell by use of a suitable current measuring device 3 serially connected in the circuit of the cells. Since the current and voltage of the cells are analog signals, an analog-to-digital converter 4 is shown to provide the computer with digital representations of cell current and voltage signals.

The cells 2 and the current measuring device 3 are connected to resistor-capacitor (RC) circuits which function to prevent extraneous electrical noise from reaching the computer, and to provide current and voltage representative signals that are averaged over a predetermined period of time while simultaneously presenting to the computer a weighted value signal representative of the most recent cell conditions relative to current and voltage. These signals are stored in the capacitors of the RC circuits, and are released to the computer in the manner presently to be explained.

The RC circuit of the current measuring device 3 is comprised of two resistors R, and a capacitor C, connected across the current measuring device. Similarly, the RC circuit for each cell 2 comprises resistors R R and R and a capacitor C. In addition, resistors R and R, attenuate the voltage from each cell to a level suitable for use by the converter 4. When the output of the current measuring device is in a millivolt range, for example, the converter 4 operates with a millivolt signal input. If the current measuring circuit provides an output signal of a voltage level comparable to that of an electrolytic cell, then a converter operative with a comparable voltage input would be used, and resistors R and R, would not be necessary.

In FIG. 4, switch pairs S, to S,, are shown connecting the capacitors C through C to their respective resistor circuits. Such switches may be sequentially operated on commands from the computer 1, as indicated diagrammatically in FIG. 4 by arrow lines SC, to complete a circuit between the capacitors and the converter 4. The switches are momentarily operated and then snapped back upon command from the computer. When the capacitors are momentarily connected to the converter, the voltages stored therein, as representative of cell current and voltage, are conducted to the converter and computer, and the computer reads these voltages and makes a calculation therefrom, and from those previously read and stored therein, to determine if the total effective resistance R of each cell is within a permissible range for operating level of aluminum chloride concentration, or ifthe a concentration is above or below the permissible range.

When the voltage signals transmitted to the computer 1 resulting from anyone of the scannings provided by the sequential switching operation of switches S to 8,, gives a total effective resistance below the level which has been precalibrated as representing the optimum operating level for concentration of aluminum chlo ride, a signal is sent, via suitable circuit lines, only representatively shown by line 5 in FIG. 4, to a feeder 6 associated with the individual cell in which the resistance has been read as too low. The feeder responds to the signal from the computer, to add aluminum chloride and thereby restore the resistance to the predetermined desired point representing an optimum operating level for concentration of aluminum chloride in the cell electrolyte.

The feeders 6 may be of any type suitable for conveying the aluminum chloride to the cells. They may be, for example, motor operated screw conveyors, or they may be gravity feed conveyors in which solenoid operated, pneumatic valves permit the aluminum chloride to be admitted to the cell in batches or slugs from a tubular conveyor, or they may be valves that admit liquid or gaseous AlCl I Though the computer shown in FIG. 4 has been described as a digital computer, and is preferred in the present invention, an analog computer will function to control aluminum chloride level. In such a case, of course, an analog-to-digital converter would not be necessary, and some of the corrective logic provided by a digital computer would be available.

Referring now to FIG. 5, a batch or slug type of feed arrangement is diagrammatically illustrated in which an output interface 10 of the computer 1 (of FIG. 4) is employed to control a relay. device 12 of a feeder actuating means 14. The computer, as explained above, calculates the total effective resistance of the cell by use of the formula given hereinabove. If this calculated resistance is within a permissible variation on either side of a predetermined or pre-established optimum operating level, no corrective action is taken by the computer. If the total effective resistance, as calculated by the computer 1, exceeds this permissible variation on the lower.side, a signal is sent from the interface 10, via contacts 15 within the interface, and lines 16, to the relay device 12 which supplies power to the feeder actuating means 14, which means, as explained above in connection with the feeders 6, may be solenoid operated pneumatic valves which supply aluminum chloride in slugs or batches. The addition of aluminum chloride is stopped via the same circuitry when the computer 1 receives a signal from the RC network (FIG. 4) associated with the cell indicating a total effective resistance value which exceeds the preplanned permissible variation on the upper side.

In FIG. 6, an alternative arrangement is shown in which an output interface of the computer 1 provides a variable speed, continuous control arrangement for feeding the cells 2. This is accomplished with a bidirection contact closure device 22 in they output interface. vWhen the system is operating with aluminum chloride being added at a predetermined optimum operating rate, if the total effective resistance as calculated from the-voltage relayed to the computer 1 from a cell 2 is greater than a predetermined optimum operating resistance, the rate of addition of aluminum chloride to the cell 2 is decreased by transmission of a signal from the interface to a bi-directional stepping motor 23 If, however, the total effective resistance of a cell 2 calculated by the computer 1 is within the aforementioned predetermined permissible variation, the contacts 22 in the interface 20 remain open so that no signal is transmitted to the stepping motor 23, thereby causing aluminum chloride to continue to be added at the predetermined optimum operating rate.

If, on the other hand, the computer-calculated total effective cell resistance is below the desired predetermined optimum resistance range, the contacts 22 in the interface are operated to direct a signal to the stepping motor 23, via line pair 26 and 27, which causes the feeder motor 32 to increase the rate at which a feeder 6 supplies a cell 2 with aluminum chloride to a preset rate higher than the aforementioned optimum operating rate.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The following examples are further illustrative of the invention.

EXAMPLE I Measurements were made every three seconds over approximately a 69-second period of operation of an aluminum chloride cell of current, voltage and resistance, with sufficient aluminum chloride being added in response to deviation below an optimum 0.00251 ohm until the optimum was restored and addition then cutoff. A substantially constant concentration of about 5.3 to 5.7 percent aluminum chloride was thus maintained in the cellbath. The following Table I records the values for current I (amperes), voltage E (volts), resistance R, percent AlCl, in the bath determined by independent measurement, using an electrical conductivity meter precalibrated to read percent AlCl and approximate times at which a computer command went to the AlCl feeder to turn on or turn off in a fourcompartment cell. In this example, the feed automatically went on when the resistance became equalto or less than 0.00250 ohm and off when it became greater than 0.00250 ohm.

TABLE I-Continued TABLE ll-Continued Current Voltage %AlCl Resistance Command to Time Feeder (amperes) (ohms) Feeder AM Volts Amps A]C]3% Status Resistancel'l 1452 11.4 5.6 0.00251 5 :49 11.9 1944 4.6 on .00210 1452 11.5 5.0 0.00251 1054 11.8 1945 4.4 On .00207 1453 11.4 5.5 0.00250 Turn on 10:52 12.1 1943 5.1 Off .0029 1454 11.4 5.5 0.00245 1103 12.3 1942 5.3 Off 00230 1453 11.5 5.7 0.00252 Turn off 11.07 12.2 1943 5.1 Off 0022s 1453 11.5 50 0.00252 1452 11.5 5.5 0.00252 10 1452 11.4 5.3 000251 1457 11.4 5.3 0.00250 Turn on It can be seen from the foregomg Table 11 that over {332 the approximately half-hour period, the aluminum 1474 1 5 0100250 chloride concentration was maintained by the use of 12;; Turn Off the AlCl concentration control procedure of the in- 15 vention at a reasonably constant optimum operating level ranging between a low of about 4.4 percent and a high of about 5.3 percent. EXAMPLE 11 While the invention has been described in terms of Aluminum was produced by electrolysis of aluminum chloride using a computer to control various levels of concentrations of aluminum chloride in the cell, for example, as described hereinabove, at a bath (solvent) composition of about 50 percent NaCl and about 50 percent LiCl. Aluminum chloride (AlCl was added and controlled at the following levels: 3.5 percent, 5 percent, 6 percent, 7 percent and 8 percent. Bath. temperature ranged from about 700 to 710 C, the period of operation being days. The current ranged from 1,000 to 3,000 amperes and the voltage from 9.8 to about 14.8. 3,000 pounds of aluminum were produced and 15,000 pounds of aluminum chloride consumed in a four-compartment cell.

EXAMPLE Ill The following Tablell shows data accumulated during minutes of a 2-month period of operation of a four-compartment bipolar cell for electrolytic production of aluminum from aluminum chloride added in response to concentration of AlCl in the cell as indicated by total effective cell resistance calculations made from measured voltage and current. The aluminum chloride concentration in the cell bath was controlled by measurement of bath resistivity during a portion of the 2- month period and by a computer control arrangement whereby the total effective cell resistance was measured the balance of the time. The computer turned the aluminum chloride feeder on and off at a predetermined total effective resistance for the cell. The feeder status of on or off at the indicated times is shown in the table. The total effective resistance for the cell was determined by the computer using the equation R (E4E)/l. The resistance and current for any given cell depends upon the size of the cell, typical cell resistance being shown in Tables 1 and 11. The value used for CEMF, E averaged about 1.95 volts and the number of compartments was four. During the period of operation represented in Table II, the feeder was turned on when R,, s 0.00209 and off when R 0.00209 ohm.

preferred embodiments, the claims appended hereto are intended to encompass all embodiments which fall within the spirit of the invention.

Having thus described my invention and certain embodiments thereof, 1 claim:

1. A method for operating a cell for the electrolytic production of aluminum from aluminum chloride which comprises:

adding aluminum chloride to the cell in response to an increase in bath conductivity signifying a depletion of aluminum chloride in the cell whereby damage to the cathode by undesired electrolysis of other ingredients in the cell is avoided.

2. The method of claim 1 wherein said increase in bath conductivity is determined by monitoring changes in the total effective cell resistance.

3. The method of claim 2 wherein said changes in total effective cell resistance are monitored by computing the said resistance from measurements of the voltage across the cell, the current, and the counterelectromotive force of the cell.

4. The method of claim 2 wherein said changes in the total effective cell resistance are computed from measured cell voltage and cell current fed thereto from the formula R [E (N E)]/l wherein R is the calculated total effective cell resistance, E is the measured cell voltage drop, E is a pre-established value for counterelectromotive force per compartment, N is the number of compartments in the cell and l is the measured current.

5. A method for operating an electrolytic cell for the production of aluminum wherein aluminum chloride dissolved in a molten salt bath is converted to aluminum metal by passing electric current through said bath, said method comprising:

a. feeding aluminum chloride into the bath;

b. monitoring the bath conductivity; 1

c. increasing the feed in response to an increase in bath conductivity; and

d. decreasing the feed in response to a decrease in bath conductivity.

6. The method of claim 5 wherein the bath conductivity is monitored by measuring the effective cell resistance.

7. The method of claim 6 wherein said measured effective cell resistance is compared to a reference resistance and wherein the rate of feed is adjusted by first decreasing the rate of addition of aluminum chloride when the measured total effective cell resistance is greater than the reference resistance and thereafter,

when the measuredtotal effective resistance for the 8. The method of claim 5 wherein said bath conductivity is monitored by current changes in the cell with the voltage across the cell held substantially constant.

9. The method of claim 5 wherein said bath conductivity is monitored by voltage changes across the cell with current to the cell held substantially constant.

10. The method of claim 5 whereby the amount of AlCl in said cell is maintained from 2-15 percent by total composition of the bath. 

1. A METHOD FOR OPERATING A CELL FOR THE ELECTROLYTIC PRODUCTION OF ALUMINUM FROM ALUMINUM CHLORIDE WHICH COMPRISES: ADDING ALUMINUM CHLORIDE TO THE CELL IN RESPONSE TO AN INCREASE IN BATH CONDUCTIVITY SIGNIFYING A DEPLETION OF ALUMINUM CHLORIDE IN THE CELL WHEREBY DAMAGE TO THE CATHODE BY UNDESIRED ELECTROLYSIS OF OTHER INGREDIENTS IN THE CELL IS AVOIDED.
 2. The method of claim 1 wherein said increase in bath conductivity is determined by monitoring changes in the total effective cell resistance.
 3. The method of claim 2 wherein said changes in total effective cell resistance are monitored by computing the said resistance from measurements of the voltage across the cell, the current, and the counterelectromotive force of the cell.
 4. The method of claim 2 wherein said changes in the total effective cell resistance are computed from measured cell voltage and cell current fed thereto from the formula Rm (E - (N X E*))/I wherein Rm is the calculated total effective cell resistance, E is the measured cell voltage drop, E* is a pre-established value for counterelectromotive force per compartment, N is the number of compartments in the cell and I is the measured current.
 5. A method for operating an electrolytic cell for the production of aluminum wherein aluminum chloride dissolved in a molten salt bath is converted to aluminum metal by passing electric current through said bath, said method comprising: a. feeding aluminum chloride into the bath; b. monitoring the bath conductivity; c. increasing the feed in response to an increase in bath conductivity; and d. decreasing the feed in response to a decrease in bath conductivity.
 6. The method of claim 5 wherein the bath conductivity is monitored by measuring the effective cell resistance.
 7. The method of claim 6 wherein said measured effective cell resistance is compared to a reference resistance and wherein the rate of feed is adjusted by first decreasing the rate of addition of aluminum chloride when the measured total effective cell resistance is greater than the reference resistance and thereafter, when the measured total effective resistance for the cell increases in response to said decreasing rate of addition of aluminum chloride, adding aluminum chloride at a rate higher than the rate prior to the initial adjusting until the measured effective resistance for the cell reaches a minimum and then begins to increase, and continuing this higher rate of addition of AlCl3 until the measured total effective resistance approaches the reference resistance, then adding aluminum chloride at a rate at least equal to the rate before said initial adjusting but less than the increased rate of addition.
 8. The method of claim 5 wherein said bath conductivity is monitored by current changes in the cell with the voltage across the cell held substantially constant.
 9. The method of claim 5 wherein said bath conductivity is monitored by voltage changes across the cell with current to the cell held substantially constant.
 10. The method of claim 5 whereby the amount of AlCl3 in said cell is maintained from 2-15 percent by total composition of the bath. 